Control valves are used in a wide variety of applications including process plants such as refineries, chemical plants, and paper plants, among many others. Control valves typically employ a control member such as a valve plug, a valve disc or other suitable control member, to manipulate a process fluid or gas flowing through a pipeline, such as a steam, water, natural gas, oil or a chemical compound. The control member is typically moved by an operating member, such as a sliding valve stem or a rotary shaft, that extends through a bore in the valve body. To prevent leakage or fugitive emissions of the process fluid or gas around the operating member, packing material is typically arranged about the operating member in the bore.
Many process applications, such as those involving aromatic or chlorinated hydrocarbons, utilize control valves that have a tendency to leak small amounts of process into the surrounding environment. Regulations governing control valve leakage have become more stringent in recent times, and in some applications, require fugitive emission concentrations to be less than 100 parts per million by volume (ppmv). Typical solutions to reduce fugitive emissions involve placing a metal bellows seal around the operating member to capture process leakage and/or exerting additional compressive stress on the packing material to create a tighter seal between the operating member and the valve body bore. However, these solutions can have undesirable effects on the operation and performance of the control valve. For instance, increasing the packing stress tends to increase the friction experienced by the operating member. The increased friction impedes movement of the operating member, which, in turn, can make it difficult to monitor and control the position of the control member. Higher levels of friction also may necessitate a larger actuator, which can increase the operating and manufacturing costs associated with the control valve. Furthermore, a metal bellows seal tends to be expensive to install, may require regular field maintenance and/or replacement, and/or may limit the travel distance of the operating member.
Thus, it is desired to provide a packing assembly for control valves which exhibits nearly zero fugitive emissions, low friction, long service life and reasonable cost.